RFR: 8221981: Simplify Map/List/Set.of() implementation

[Claes Redestad]

Hi,

current implementations of the empty static factories (Map.of(), ...)
delegate to static methods in java.util.ImmutableCollections. This
indirection isn't really useful and adds a very small startup cost.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8221981
Webrev: http://cr.openjdk.java.net/~redestad/8221981/open.00/

Testing: tier1-2, verified a tiny improvement in startup profiles

Thanks!

/Claes